1. Field of the Invention
The present invention broadly relates to measuring halation in transparencies, such as aircraft windscreens made of plastic or the like, and, more particularly, is concerned with a method for measuring a haze index of transparent materials that can be related to contrast loss when viewing a target through the material.
2. Description of the Prior Art
The most common reasons for removing an aircraft transparency are optical, and the most common optical problem is haze, halation or contrast loss as perceived by the pilot.
The presently accepted method of measuring haze is based on a technique developed at the National Bureau of Standards (NBS). This method has been adopted as a haze measurement standard by the American Society for Testing and Materials (ASTM) and by the U.S. Government. The Gardner Haze Meter is a specific device used to measure haze, since its design is based on this method.
By way of background, light incident on a transparent material can be absorbed, reflected, scattered and transmitted. Since light (energy) must be conserved, the quantities of absorbed, reflected, scattered and transmitted light must add up to the amount of incident light. The scattered and transmitted light are the two parts of interest for measuring haze using the NBS method. In this case the total light that passes through the transparent material is equal to the amount transmitted plus the amount scattered. Only the transmitted portion is usable to form an image of the object from which the light originated; the scattered light has lost its image forming information.
The NBS definition of haze is the ratio of the scattered light to the total light that comes through the transparency (S+T). In equation form: ##EQU1## where: H=haze
S=scattered light PA1 T=transmitted (image forming) light PA1 M=mean luminance of white and black target areas (without the transparency) which is equal to (W+B)/2.
Values of haze range from 0 (no scattering) to 1 (total scattering). Although the definition seems quite reasonable and the haze values are bounded and well behaved, it is not possible to directly relate the haze value to contrast loss and visual performance degradation. In addition, the instrumentation is designed specifically for relatively small, thin, unscratched, flat samples that can be placed flush with the entrance aperture of the integrating sphere used to make the measurements. The measurements are invalid if made at any angle other than the normal to the surface. Furthermore, the instrumentation is not usable for measuring the haze of an aircraft transparency while it is installed on the aircraft (or under field conditions).
Because of these significant disadvantages, a new technique for measuring and defining haze is needed that can be directly related to contrast loss and observer performance and can be used to measure windscreens while they remain installed on the aircraft.